1. Field of the Invention
The present invention relates to, for example, a vibration damping device used for an automobile engine mount or the like, and particularly to a fluid-filled type vibration damping device that utilizes the flow behavior of the fluid sealed inside.
2. Description of the Related Art
Fluid-filled type vibration damping devices are known as one type of vibration damping devices used as automobile engine mounts, sub-frame mounts and the like. These conventional fluid-filled type vibration damping devices includes: a first mounting member; a second mounting member; a main rubber elastic body connecting the first and second mounting members; a pressure receiving chamber and an equilibrium chamber filled with a non-compressible fluid; and an orifice passage by which the pressure receiving chamber and the equilibrium chamber communicate with each other permitting fluid flows through the orifice passage.
Meanwhile, these fluid-filled type vibration damping devices have suffered from the problem that abnormal noise will occur due to cavitation upon input of impulsive large amplitude vibration such as when an automobile rides over a bump. Specifically, when marked negative pressure is applied to the pressure receiving chamber due to input of a shocking, large amplitude vibration, air bubbles due to cavitation are generated in the pressure receiving chamber, and shock waves are emitted when the air bubbles disappear. Then, by those shock waves being transmitted to the vehicle, passengers hear this as noise.
To reduce or prevent this kind of cavitation noise, U.S. Publication No. US 2010/0072683, for example, proposes a structure provided with a leak passage (short passage) which opens when marked negative pressure occurs in the pressure receiving chamber. Namely, U.S. Publication No. US 2010/0072683 discloses that a leak passage is formed on a partition member, and also a relief valve (valve member) is formed as an integral unit with a moveable film to open and close the leak passage, and when negative pressure of a level for which cavitation will occur is applied to the pressure receiving chamber, the relief valve is deformed and the leak passage is opened. With this arrangement, the flow of fluid through the leak passage reduces the negative pressure of the pressure receiving chamber, and the noise due to cavitation is reduced or prevented.
However, according to the structure proposed by U.S. Publication No. US 2010/0072683, the relief valve is a thin film form having a uniform thickness entirely. Therefore, when negative pressure occurs in the pressure receiving chamber, the entire relief valve easily becomes greatly deformed, and the leak passage is in a communicating state. This causes escape of the pressure fluctuation of the pressure receiving chamber through the leak passage when vibration is input. Additionally, even if the relief valve is maintained in a shut off state, the pressure fluctuation of the pressure receiving chamber escapes to the equilibrium chamber side due to the elastic deformation of the relief valve itself. As a result, the fluid flow volume through the orifice passage decreases, possibly causing adverse effect on the vibration damping effect characteristics exhibited by the orifice passage.
It is conceivable that the deformation spring characteristics would solidify by increasing the relief valve thickness, but in conjunction with the thick walled part of both sides of the circumferential direction of the relief valve being formed integrally and being restrained, it is very difficult to adjust the deformation spring characteristics of the relief valve to a suitable value. Moreover, a thin film form relief valve is formed projecting from the outer circumference end surface of the maximum thickness fixed part formed on the outer circumference edge part of the moveable film. Accordingly, cracks or damage are likely to occur due to marked stress concentration on the base part of the relief valve, making it difficult to ensure durability and reliability.